I need to create a subscription module on magento. I created a module call subscription under a custom namespace in the local folder. I have register it with magento, so, I'm good to go. I want to be able to extend core Catalog_Product and add a couple new fields that relate to subscriptions, like trial period, charge date, and other subscription attributes. I also, need to create a way to manage the users that are subscribed to these subscriptions. What would be the best approach for this?